For cases for exhaust-gas purifying catalysts (converters) or cases for mufflers, which are mounted to automobile exhaust systems of automobiles, pipes having a large diameter are used as raw material in order to increase capacity. Further, an end portion of the case member is provided with a tapered portion for connection to a forward or rearward member, and is optionally further, provided with a straight pipe portion having a smaller diameter that is continuous with the tapered portion.
FIG. 1 illustrates an example of such a case 1 including: a pipe portion (1c) having a large diameter as a raw material; tapered portions (1b) provided for connection to forward and rearward members; and straight pipe portions (1a) each having a smaller diameter and being continuous with the tapered portion.
As a processing method, the spinning method as illustrated in FIG. 2 has been used in many cases.
Further, for the purpose of suppressing wrinkles at the time of processing, adjusting the sheet thickness of the work piece, or improving dimensional accuracy of the inner and outer surfaces to be processed, for example, a method using a core body as illustrated in FIG. 3 has been also carried out.
However, when performing the processing with a large number of passes, the core body functions effectively only at the final pass, but barely functions in the midway process. Further, for reasons such as requiring a jig for holding the core body, the technology using the core body when performing diameter reducing processing on the pipe end is not necessarily practical.
Therefore, when performing spinning on the pipe end for reduction of the diameter thereof, processing has been often performed without using the core body while pressing rollers onto a pipe body in a hollow state.
In the case of performing the spinning without using a core body, in order to perform the processing without a problem, it is necessary to maintain the original shape of the pipe body against the processing force applied by the rollers.
However, for such an article obtained by processing the pipe body, there are demands for reduction in thickness of a pipe wall for the purpose of reducing weight, etc. When the pipe wall is reduced in thickness in comparison with the diameter of the pipe body, rigidity of the pipe body is insufficient. As a result, the shape accuracy of the article obtained by processing the pipe body is reduced. Further, when the work piece pipe body is elastically bent or corrugated during the spinning, the rollers collide against the deformed portion, and hence wrinkles or deformations occur in regions subjected to the spinning, with the result that it is impossible to continue spinning. In other words, a defective article as illustrated in FIG. 4 is formed.
Such a phenomenon where shape accuracy is reduced and the processing cannot be continued is more likely to occur as the thickness of the work piece pipe body becomes smaller or as processing speed becomes faster.
In this context, JP 2006-346695 A proposes the following. In order to suppress elastic deformation resulting from the insufficient rigidity of the work piece pipe body, prevent occurrence of wrinkles and cracks, and increase shape accuracy and processing speed, when performing the spinning for reducing a rotating metal pipe body without using a molding die while pressing the processing roller on the outer surface of the pipe body, a filler made of a foam resin is interposed inside the pipe body, and the pipe body is subjected to the diameter reducing processing by the processing roller together with the filler.